This invention relates generally to chemical sensors for toxic gases, and in particular to carbon monoxide sensors.
Sensors and sensing systems for detecting toxic pollutants are gaining increasing prominence in process control, residential environment, transportation vehicles, and in work places. Carbon monoxide is an odorless poisonous gas, with an exposure limit of only 35 ppm.
Solid state sensing devices impart numerous advantages. In particular, solid state sensors are user friendly, possess extended shelf life and operational life, can be easily mass produced, and reduce the risk of improper handling by the user.
The prior art for detecting carbon monoxide colorimetrically involves the use of palladium and molybdenum compounds, as described by M. Shepherd, Anal. Chem. 19 (2), 77, (1947). The use of these compounds have also been reported as early as 1910 by C. Zenghelis, Z. Anal. Chem., 40, 429, (1910) and the literature was reviewed in 1935 by J. Schmidt, "Das Kohlenoxyd", Akad Verlag, Leipzig, P 186, (1935). This chemistry also appears in "Spot Tests in Inorganic Analysis" by F. Feigel, V. Anger, R. Oesper, Elsevier Publishing Company, New York, P. 168 (1972).
U.S. Pat. No. 3,112,999 to Grosskopf is directed to a solid state carbon dioxide sensing device.
The basic chemical reactions for the palladium catalyzed reduction of molybdenum by carbon monoxide are as follows: EQU Mo.sup.+6 +CO.fwdarw.Mo.sup.+3 +CO.sub.2 ( 1) EQU Pd.sup.+2 +CO.fwdarw.Pd.sup.0 +CO.sub.2 ( 2) EQU Pd.sup.0 +Mo.sup.+6 .fwdarw.Pd.sup.+2 +Mo.sup.+3 ( 3)
The reaction of carbon monoxide with molybdenum (Equation 1) is very sluggish. Therefore, palladium is employed as a catalyst, where palladium is first reduced by carbon monoxide. Reduced palladium, Pd.sup.0, in turn then reduces the molybdenum to lower oxidation states the most common one being Mo.sup.+3, which is also known as "molybdenum blue". Thus, a slightly yellow solution is changed to a blue color. The intensity of the blue color directly correlates to the extent of CO exposure. Unfortunately, however, the reduced molybdenum is rather stable, and does not quickly go back to the initial oxidation state so that the same chemistry could be recycled. This irreversibility makes this chemistry of limited use.
A reversible CO sensor is shown by Shuler et al, U.S. Pat. No. 4,043,934 which has a Mo, W or V color forming agent, Pd catalyst and Cu, Ni or Fe reversing agent. The sensing reagent is deposited on an inert carrier which is hydrophilic or contains water or OH groups, e.g. silica gel, alumina, polymeric alcohol, polyglycol, cellulose, glass wool and sponges.
M. K. Goldstein in U.S. Pat. No. 5,063,164 describes a biomimetic sensor for detecting the presence of airborne toxins including CO. That patent suggests several recipes for making regenerable sensors, but does not address the criteria or requirements for a successful reversible sensor; nor does it address the chemistry or mechanisms to make the CO sensor completely specific.
Goldstein shows a solid state CO sensor having five components: (1) palladium salt, (2) molybdenum and/or tungsten salt or acid salt, (3) copper salt, (4) cyclodextrin molecular encapsulant which encapsulates at least one but not all of the other components, and (5) chloride salt, all impregnated into a porous substrate. The Mo,W/Pd/Cu system is as in Shuler. The improvement is the encapsulant which extends sensor lifetime. An excess of chloride ions are also provided to extend lifetime. The substrates include silica-gel beads and porous glass, in which diffusion of gases can be rather slow.
Goldstein's patent does not reveal (a) how fast the reverse reaction occurs, or (b) whether it can stand a drastic environment like 100% CO. U.S. Pat. No. 5,063,164 uses Cu.sup.2+ salts as the reversing agent. Cu.sup.++ and Cu.sup.+ ions are very stable at ambient atmospheric conditions. Therefore, the Cu.sup.++ /Cu.sup.30 pair does not fully meet the criteria of a successful reversing agent for a deadly toxic gas like carbon monoxide.